\documentclass[]{article}
\usepackage{epic}	% required by ecltree and fancybox packages
\usepackage{ecltree}	% to draw the GP trees
\usepackage{fancybox}	% required by \Ovalbox

\begin{document}

% minimum distance between nodes on the same line
\setlength{\GapWidth}{1em}    

% draw with a thick dashed line, very nice looking
\thicklines \drawwith{\dottedline{2}}   

% draw an oval and center it with the rule.  You may want to fool with the
% rule values, though these seem to work quite well for me.  If you make the
% rule smaller than the text height, then the GP nodes may not line up with
% each other horizontally quite right, so watch out.
\newcommand{\gpbox}[1]{\Ovalbox{#1\rule[-.7ex]{0ex}{2.7ex}}}

\begin{bundle}{\gpbox{+}}\chunk{\gpbox{y}}\chunk{\begin{bundle}{\gpbox{+}}\chunk{\begin{bundle}{\gpbox{*}}
\chunk{\gpbox{x}}\chunk{\gpbox{y}}\end{bundle}}\chunk{\begin{bundle}{\gpbox{*}}\chunk{\gpbox{y}}
\chunk{\begin{bundle}{\gpbox{*}}\chunk{\gpbox{x}}\chunk{\gpbox{x}}\end{bundle}}\end{bundle}}\end{bundle}}
\end{bundle}
\end{document}

